1. Field of the Invention
The present invention relates to an ink-jet head which does printing on a printing medium such as paper or cloth by discharging ink, and a printing apparatus using the head.
2. Related Background Art
A printing apparatus such as a printer, copier or facsimile is constituted such that it prints a dot-pattern image on a printing material based on image information. The above printing apparatuses can be classified into an ink-jet system, a wire dot system, a thermal system, a laser beam system and the like, according to printing system thereof. Of these, the ink-jet system has an ink-jet head, and the head has an energy conversion means for generating discharge energy used for discharging ink to a fluid path. The head is constituted such that it leads ink from an ink supply port to the above fluid path via a fluid chamber, jets the ink toward a printing material as flying droplets by the discharge energy given by the energy conversion means and does printing by the ink droplets struck the printing material. Of these, an ink-jet head which discharges ink by using thermal energy has been practically used since it has such advantages that ink discharge ports for forming flying droplets by discharging ink droplets for printing can be arranged at high densities and that a reduction in the overall size thereof can be easily done. In addition, in recent years, the number of nozzles arranged in the ink-jet head has been increasing in response to the demand for high-speed printing.
However, since the ink-jet system deals with fluid ink, the meniscus vibrations in the discharge nozzles are greatly disturbed by the vibration of the ink, whereby deterioration in the quality of an image may occur. Particularly, in the case of an ink-jet head having a number of nozzles arranged at high densities therein, since the amount of ink moving per unit of time is large, the inertial force of the ink in a tank system which works to move the ink forward (toward the head) when the discharge of the ink is ceased also become large. Because of this inertial force, a positive pressure is exerted on the nozzle, whereby meniscus is protruded. When the following print signal comes in at this point, small ink droplets are splashed and so-called xe2x80x9csplashedxe2x80x9d printing results.
FIG. 12 is a graph showing the vibration waveform of the pressure in an ink flow path to a discharge pulse when a predetermined discharge is made by an ink-jet head. FIGS. 13A, 13B and 13C are cross-sectional view of a nozzle showing the states of meniscus in the section A (before discharge), the section B (during discharge) and the section C (immediately after discharge stop). As shown in FIG. 12, the amplitude a of the vibration of the pressure in a flow path after discharge stop is large and the pressure in the flow path is a positive pressure, and this vibration disturbs meniscus vibration at the next discharge. Specifically, in the section A in FIG. 12, a stable meniscus M is formed as shown in FIG. 13A. When discharge is made (or heating element 53 is pulse-driven) as in the section B with the meniscus M in this state, a good droplet 50 is produced as shown in FIG. 13B. Then, in the section C immediately after the discharge, the pressure in a flow path 52 is greatly inclined to a positive pressure by the inertia of a fluid moving toward a discharge port 51, and the meniscus M is formed in such a state that it protrudes from the discharge port as shown in FIG. 13C and, at the worst, ink drips from the discharge port 51. Therefore, as described above, when the discharge of ink is resumed in the state of FIG. 13C, small ink droplets are splashed and a good image cannot be formed.
As a method for overcoming such phenomena, it is practiced that flow resistance is adjusted by altering the diameter of a filter or the flow path of ink so as to control the meniscus vibration. However, when the flow resistance is set to be large, a sufficient supply (refill) of ink will not be able to be provided to discharge nozzles eventually and a sufficient discharge amount cannot be obtained at the time of discharge, thereby causing insufficient concentration. On the other hand, when the flow resistance is set to be small, a sufficient supply of ink can be provided but the amplitude of the meniscus vibration cannot be controlled, whereby the flexibility in designing the ink-jet head is quite limited. As another method, there is a method (Japanese Patent Application Laid-Open No. 6-210872) in which pressure vibration is absorbed by providing a buffer chamber for keeping bubbles in a common fluid chamber.
Although this method is perfect as a means for suppressing the pressure vibration, flexibility is hardly left in the volume and form of the buffer since the buffer is provided in the common fluid chamber. Further, since bubbles are present in the vicinity of nozzles, the growth of the bubbles increase the possibility of the occurrence of an ink discharge failure.
The present invention has been invented to solve the above problems. It is an object of the present invention to provide an ink-jet recording head which suppresses unstable ink discharge caused by the vibration of ink which occurs when the ink is discharged, and a printing apparatus using the ink-jet recording head.
To achieve the above object, the present invention proposes an ink-jet recording head comprising a printing head unit which has one or more discharge portions for discharging one or more types of fluids for printing, a tank holder unit in which one or more tanks for storing one or more types of fluids to be discharged by the printing head unit are loaded, and a fluid supply path which is formed in the tank holder unit and communicates with the printing head unit and the tank, wherein a buffer chamber for keeping gas is connected to the fluid supply path.
The above fluid supply path and the above buffer chamber are preferably formed by joining one or more supply path-forming members to the above tank holder unit.
Further, the above fluid supply path is preferably formed in the direction perpendicular to the direction of gravity.
Still Further, the flow path which connects the above buffer chamber to the above fluid supply path is preferably disposed at an angle of at least 90xc2x0 from the direction in which a fluid is headed from the tank toward the discharge portion.
Still Further, the cross section of the flow path which connects the above buffer chamber to the above fluid supply path is preferably smaller than that of the buffer chamber.
In addition, the inner wall of the above buffer chamber is preferably made water-repellent.
In the above printing head, the above discharge portion preferably has a flow path which communicates with a discharge port for discharging a fluid, a thermal energy-generating element which is provided along with the flow path and generates thermal energy for discharging a fluid, and a common fluid chamber for supplying a fluid to the above flow path.
Further, the present invention also proposes a printing apparatus using the above ink-jet recording head, which discharges droplets from discharge portions toward a printing medium to do printing.
According to the above constitutions, in the ink-jet recording head in which the fluid supply path which communicates with the tank and the discharge portions for discharging the fluid in the tank is formed by joining the printing head unit having the discharge portions to the tank holder unit and the printing apparatus using the head, by connecting the buffer chamber for keeping gas to the above fluid supply path, the vibration of the pressure in the flow path due to the vibration of the ink at the time of discharging the ink can be suppressed, a stable discharge state can be maintained, and an image of high quality can always be obtained.